Fly-ash reinjection



June 19, 1956 E. c. MILLER ET AL 2,750,903

FLY-ASH REINJECTION 2 Sheets-Sheet 1 Filed May 22. 1.952

INVENTORS EARLE 6. Nu. A El? L 1/ fi/vofisws Md w- ATTORNEY June 19, 1956 E. c. MILLER ET AL 2,750,903

FLY-ASH REINJECTION 2 Sheets-Sheet 2 Filed May 22. 1952 INVENTORS [ARLE C. N/LLE/i' A V ANDREWS BY Mm Maggi? ATTORNEY United States Patent signors to Riley Stoker Corporation, Worcester,Ma'ss.,-

a corporation of Massachusetts Application May 22, 1952, Serial Ne. 289,485 8 Claims. (Cl. 110--165) This invention relates to ily-ash reinjection. and more particularly toan apparatusfor causing fly-ash collected in different parts of a steam generating unit to be passed through the combustion chamber for more complete combustion.

It is usual practice in the operation of many steam generating units to collect fly-ash and cinder in hoppers and dust collectors situated in the boiler passesand just preceding the stack. This is reinjected into the combustion chamber because of the high percentage of unburned carbon in this fly-ash, which would cause the unit to be very inefficient if the carbon were not burned out more compl'etely. The reinjection of fly-ash brings about certain problems, however, and" the principal one of these is that continual recirculation of the fly-ash means that many particles which have very little carbon are also recirculated. Reinjection of such particles increases the dust load in the furnace and boiler passes which, in turn, increases clinkering. on the grate, slagging on the walls, anderosionof. the tubes. It also increases dust emission from thestack by increasing the dust load on the collector. Hence, it is better not. to reinject particles having a low carbonpercentage. Also, in determining the amount of the fiy-ash collected which can be reinjected, the operator is limitedby the capacity of his reinjection system; if more fly-ash is being collected than can be carriedback to the furnace by the reinjectionsystem, the excess fly-ash will begin to accumulate and present aproblem of disposal. Also, when the capacity of the reinjection system or of the furnace is such that it is possible to reinject only' a portionof the collected fiy-ash, it is desirable to provide a means to assure that the portion that is reinjected contains the particles of high carbon content. The present invention overcomes many of these problems of the prior art ina novel manner.

It is therefore an outstanding object of the present invention to provide a fi-y-ash reinjection system which returns only high carbon-contact particles to the furnace.

It is a further object of the invention to provide a reinjection apparatus which provides for the disposal of all fly-ash in excess of the amount which itcan handle conveniently.

Another object of the present invention is the provision of a fly-ash reinjection apparatus which disposes of particles except those of high carbon content and which provides for the disposal of amounts of fiy-ash in excess of that which the system can handle conveniently. I

A further object of this invention is the provision of an apparatus for separating light particles of fly-ash: from heavy particles of fly-ash. I,

Another object of this invention isthe provision of apparatus for separating large, high-carboncontent particles of fly-ash from smaller, burned-out particles of fly-ash.

A still further object of the present invention is the provision of apparatus for separating large and non-magnetic particles of fly-ash from small and magnetic particles thereof.

It is another object of the instant invention to provide an apparatus for separating the magnetic particles of flyash having low carbon content from the non-magnetic particleswhich have large carbon content.

It is a further object of this invention to provide an apparatus for disposing of amounts of fly-ash being'trans "ice mitted to a reinjection system, which amounts are in excess ofapre-determined volume of transmittal.

Although the novel features which are believed to be characteristic of this invention will be particularly pointed out in the claims appendedhereto, the invention itself, as to its objects and advantages, the mode of its operation and the manner of its organization may be better understood by referring to the following description. taken in connection with the accompanying drawings forming a part thereof, inwhich:

Figure 1 is a longitudinal sectional view of a steam generating unit embodyingthe present invention,

Figure 2 is a vertical sectional view'of an apparatus within the concept of the invention,

Figure 3 is a view of the apparatus shown inv Figure 2 taken on the line 33 thereof,

Figure 4 is a vertical sectional view of a variation of the apparatus shown in Figure-2,

Figure 5 is a vertical sectional view of another appa ratus,

Figure 6 is. a vertical sectional view of a variation of the apparatus shown in Figure 5,

Figure 7 is a vertical sectional view of another variation of the device shownin Figure 5,

Figure 8 is a vertical sectional view of another variation of the apparatus shown in Figure 5, V

Figure 9 is a vertical sectional view of an apparatus within the scope ofrthe present invention,

Figure 10 is a vertical sectional view ofa variation of the device shown in Figure 9, and

Figure 11 is a vertical sectional view of still another variation of the device shown in Figure 9.

Referring first to Figure 1, wherein is shown tov best advantage the general aspects of the present invention, a steam generating unit, designated bythe reference numeral 29 is, shown ashaving a furnace 21 and a boiler 22. The furnace 21 is of the traveling-grate, spreader-Stoker type and discharges at the rear. The traveling grate 23 extends along the lower portion of. a combustion chamber 24 and discharges into an asli pit 25. A spreader stoker 2 6 sprays coal or other fuel over the surface of the traveling grate and the products of combustion pass upwardly through the combustion chamber 24 intothe boiler 22. The boiler 22 comprises a steam drum 27 from which steam is taken, a water drum 28 lying therebeneath, and a header 29 situated in the upper forward wall of the combustion chamber 24. The drums and the header are joined by water tubes in a well-known manner to obtain proper circulation of the water. Suitable bafiles provide for the flow of the gases over the tubes into an outlet duct 30 extending from the upper rear portion of the furnace. The duct 30' connects with a dust collector 31 of a well-known typev and the collector, in turn, discharges gas, relatively free of flya'sh, into a duct 32 leading to the stack 33. A damper 34 is mounted in the duct 32 for control of gas flow.

A hopper 35' extends downwardly from the lower portion of the boiler adjacent the water drum 28. and at a section of the boiler wherethe gas is subjected to a degree reversal. A certain amount of fly-ash and cinder may be thrown into the hopper 35 by' the gas reversal and this collected material maybe. returned to the furnace. y heans of an ejector apparatus 36 and a nozzle 37. Flow of material from the hopper 35 may be prevented by use of a slide valve' 38 in the bottom of the hopper and ejector operation may be regulated by means of the valve 39 in the ejector line.

The collector 31 operates to separate the fly-ash and cinder particles from the gas and to permit the gas to flow onward to the stack while retaining the fly-ash in its downwardly-extending'hopper 40. The hopper 40 narrows in the lower portion and is provided with a slide door or valve 41. The bottom of the hopper is connected with a c3 vertical duct 42 of a generally square cross-sectional shape. The lower end of the duct 42 is connected to a gravity separator 43 whose function is to separate the heavier particles from the lighter particles. Such an apparatus is shown in Figure 2 and will be described more fully hereinafter. The separator 43 is provided with two downwardly extending outlet branches. Branch 44 carries the lighter particles and is connected with a duct 45 which leads ultimately to a disposal means such as the ash pit 25 in a manner to be described more fully hereinafter. Branch 46 of separator 43 carries the heavier particles and is connected to the inlet of a magnetic separator 47 whose function is to separate the low-carbon content, magnetic particles from the high-carbon, nonmagnet particles. Such an apparatus is shown in Figure and will be described more fully hereinafter. A branch 48 containing low-carbon, magnetic particles leads downwardly and ultimately discharges to the ash pit through apparatus to be described hereinafter. A branch 4% carrying the high-carbon particles extends downwardly to a limiting apparatus 50 whose function is to discharge to the ash pit any amounts of fly-ash in excess of the amount which can be conveniently handled by the reinjection apparatus. Such a limiting apparatus is shown in Figure 9 and will be described more fully hereinafter. An overflow duct 51 discharges to the ash pit and a main duct 52 leads downwardly to an ejector 53. The ejector 53 is rendered operative by the introduction of steam or air under pressure through a valve 54. The well-known action of the ejector carries particles out of the duct 52, mixes them with the operating fluid, carries them along a conduit 55, andprojects them through a nozzle 56 into the furnace. The ducts 45, 48, and 51 are connected to a collecting manifold 57 which discharges through a conduit and a valve 58 into an ejector 59. The valve 58 is of a Well-known rotary drum type which will permit the passage of fly-ash downwardly with permitting the passage of any appreciable amount of gas upwardly. The ejector is operated by steam or air which is under the control of a valve 60 and discharges the fly-ash through a conduit 61 into the ash pit 25.

The operation of the apparatus will be evident from the above description. The spreader stoker 26 distributes fuel over the traveling grate 23. Some of the fuel burns in suspension and the remainder burns on the surface of the grate. The ash on the grate is discharged into the ash pit 25. Some of the ash and cinder is in suspension in the gases and is carried upwardly through the combustion chamber 24 and then through the boiler 22. Some ash falls into the hopper 35 and is reintroduced into the combustion chamber through the nozzle 37. The ash falling into the hopper 35 tends to consist of larger particles contained in the gases and has a high percentage of carbon. The gases then pass out of the boiler passes and go through the duct into the dust collector 31. The fly-ash separated from the gases by the collector goes downwardly into the gravity separator 43. The small, dense particles, which, as has been explained above, have little remaining carbon, are discharged to the ash pit through the duct 45, the manifold 57, the gas valve 58, the ejector 59, and the conduit 61. The larger, heavier particles, which contain high percentages of carbon, continue onwardly through the branch 46 into the magnetic separator 47 At the separator 47 the magnetic particles, which have a low carbon content, are forced to go downwardly through the duct 48 and to be discharged into the ash pit 25. The non-magnetic particles, which have high carbon content pass downwardly through the branch 49 into the limiting apparatus 50. From there the ash passes into the duct 52 and is reinjected into the furnace through, the ejector 53, the conduit 55, and the nozzle 56. After the capacity of the reinjection system, i. e., ejector 53, conduit 55, and nozzle 56, has been met, any excess of fly-ash is discharged down the duct 51 and carried to the ash pit for discard.

Several alternative procedures are possible within the scope of the present invention. One possibility is that all ash-collecting apparatus in the steam generating unit be connected to the same apparatus or type of apparatus has been described above in connection with the dust collector 31. In other words, the hopper could very well be connected to the duct 42, so that its ash could be subjected to a segregation treatment before being reinjected into the furnace; on the other hand, the hopper 35 could be provided with a segregation apparatus of its own. Another alternative arrangement is to have the duct 51 from the limiting apparatus discharge into a storage system instead of into the ash pit; this system would be particularly advantageous where there are large fluctuations in load, whereby the dust collector would not be collecting as much ash during low-load periods as during high-load periods of operation.

Referring next to Figures 2 and 3, wherein is shown the gavity separator 43. The separator 43 has a vertical section '71 which merges with an inclined section 72. The inclined section 72 in turn merges with another vertical section '73 which is displaced somewhat from the vertical section 71. The lower wall of the inclined section 72 is provided with a riffle 74 which consists of a series of downwardly extending troughs 75 arranged in side-byside relation. The troughs 75 taper in size from a very small portion adjacent the intersection of the vertical portion 71 with the inclined section 72 to a large portion overhanging the vertical section 73. The vertical section 73 is divided into two branches 76 and 77 and a movable vane 78 extends upwardly between them to permit selective separation of the flow of particles downwardly from the riffie 74. During the operation of the apparatus, the ash from the dust collector or the like passes downwardly through the vertical section 71 and falls on the riffle 74. The nature of the riifle is to cause the dense, small particles to flow along the bottom of the troughs 75, while the less dense, larger particles flow in the upper portions. Thus, the ash is segregated or classified and it is only necessary to set the vane 78 in the proper position to cause the desired segment of larger particles to fall into the branch 77, while the smaller particles fall into the branch 76 for ultimate discard.

In Figure 4 is shown an alternative construction of the gravity separator described above. The separator 80 is provided with a vertical section 81 beneath which is arranged a hopper 82 tapering downwardly to a connection with a branch conduit 83. In a position offset from the vertical portion, but arranged in side-by-side relation to the hopper 82, is a hopper 84 which tapers downwardly to a connection to a branch conduit 85. The adjacent top edges of the hoppers 32 and 84 meet and form a tentlike configuration from the ridge of which rises a vane 86 which extends toward the hopper 84 and which is generally in the plane of the adjacent side of the hopper 82. During the operation of the apparatus, the ash particles fall downwardly through the vertical section 81. The smaller particles are caused to flow around the vane 86 into the hopper 84 and the branch conduit by the suction on the conduit 85 brought about by the action of the ejector to the ash pit. The larger particles, not being as easily deflected from their downward paths, fall downwardly into the hopper 82 and the conduit 83. The conduit 83 leadst o-the reinjection system to the furnace, while the-conduit 85 leads to the ash pit.

Referring next to Figure 5, wherein is shown the magnetic separator 47 suitable for use in the practice of the present invention, the separator 47 consists of a vertical section 91 merging into an enlarged intermediate section 92.providing a portion 93 which is olfset from the vertical section 91. Under the intermediate section are two branches 94 and 95 with branch 94 underlying the offset portion 93 and the branch 95 underlying the vertical portion 91. Avvane or divider 96 extends upwardly from the intersection of the adjacent walls of the two branches.

An electromagnet 96 having. a core 98 and windings 91' connected to a suitable source of electrical power (not shown) is situated adjacent the walli of the offset portion 93 with its polarity such that magnetic particles will be attracted to the end? of the core adjacent the oifset portion 93. In the operation of this apparatus, the ash enters downwardly through the vertical section 91'. I The nonmagnetic particles, whichhave a high-carbon contentpass downwardly into the branch 95 and thence to the reinjection system to the furnace. The magnetic particles, containing relatively low amounts of carbon are deflected by the attraction of the magnet into the offset portion 93 and fall into the branch 94 which leads: to: the ash. pit. Means, not shown, is provided for periodically interrupting the current flowing in the windings of the electromagnet in order that a thick layer of particles will not build up on the duct wall.

Figure 6 shows an alternative magnetic separating device which may be used in the practice of the present invention. The separating device 100 consists of a substantially vertical duct 101 which merges into two similar branches 102 and 103', there being a pointed junction between the two branches exactly in the middle of the vertical duct 101. A downwardly extending vane 104 is formed integrally with the wall of the vertical duct overlying the branch 103 and the vane extends at an angle to the side wall with its outward end overlying the pointed junction. Externally of the wall towhichthe vane 104 is attached and at a lower level, is an electromagnet 105 having a core 106 and windings 107 suitably energized. The particles fall downwardly inthe vertical duct 101 and are deflected by the vane 104 into the half of the duct beneath which the branch 102' lies. The electromagnet 105,. however, attracts the magnetic particles and. they move back under the vane into the half of the duct beneath which the branch 103 lies. The magnetic particles proceed downthe branch 103 to the ash pit and the non-magnetic particles go down the branch 102' to be reinjected into the furnace.

In Figure 7 is shown another version of a magnetic separator which may be used in connection with. the invention. The separator 110 consists of a horizontal duct-111 having a branch 112 extending therebeneath at an angle thereto. An electromagnet 113 having a core 114 and suitably energized coils 115 is mounted on the underside of the duct 111 adjacent the branch 112. The ash enters the duct 111 from the left in the illustration and is carried by a current of air due to the respiratory eifect of the ejector of the reinjection system. The non-magnetic particles continue in a straight-line path through the duct 111 and are reinjected into the furnace. The magnetic particles are drawn toward the side of the duct 111 from which the branch 112 leaves. The intersection of the duct 111 and the branch 112 is such that the sharp edge formed at the acute angle of the intersect-ion is raised considerably above the edge at the obtuse angle. This fact and the fact that the ash pit ejector produces a certain amount of suction on the branch 112 tends to take off the magnetic particles previously classified by the elcctromagnet' 113.

In Figure 8 is shown still another alternative form of magnetic separator which may be used in the practice of the present invention. The separator consists of a straight, horizontal duct 121 which terminates inits junction with a vertical duct 122. Extending axially ofthe duct 121 and through the wall where it joins theduct 122 is a branch 123. Electromagneticcoils124 suitably energized are wrapped around the duct 121 toproduce an electromagnetic flux pattern concentric with the axis of the duct 121. Flyash enters the device from the left in" the illustration, being carried by a slight gas flow caused by the reinjection system ejector. The magnetic particles are forced outwardly of the duct adjacent the wall thereof and eventually pass into the duct 122 for discharge into the ash pit. The branch 123 takes the particles which are closest to the axis of the duct 121, which particles are mostly'nonmagnetic and of high carbon content. In this particular apparatus andiirnsomeiofi the others: described, it is not, of course, to' be hoped that the classification into magnetic and non-magnetic is perfectly accomplished; all that can'behoped for isthat the fractions of thefi'y-ash that are reinjected to the furnace. will be considerably higher in carbon content. than the average of the fly-ash collected in the: boiler passes and dust'collector.

In Figure 9 is shown the limiting apparatus 50 for use with the invention. The apparatus 50" consists of a vertical conduit 131 which forms a Y with two branch conduits 132 and 133' which extend at equal angles to the axis of the conduit'131'. The intersection of the branches 132 and 133 form a pointed edge to which an adjustable vane 134 is hingedly attached; The ash comes down the conduit 131 on its way to the reinjection system to the furnace. The vane position is; selected so that a predetermined portion of. the ash is deflected into the branch 132 for discharge into the ash pit, while the remainder is permitted to continue onward to' be reinjected into the furnace. By observing the how of ash into the furnace and by adjustingzthe position of the vane 1-34, the operator may select the proportion of the ash which is reinjected.

Figure 10 shows another limited apparatus within the scope of the present invention. The apparatus consists of avertical duct 141 having a hopper bottom 142 from the center of which rises a standpipe 143 which is situated axially of the duct 141. The top of the standpipe is provided with a telescoping sleeve 144 whose position on the standpipe is adjustable. The lower portion of the standpipe extends through the hopper bottom 142 and connects to a conduit 145 leading tothe ash pit. Extending'from the duct 141 at a level somewhat below the top of the standpipe are two branch conduits 146 and 147 the outward ends of which are connected't'o ducts 148 and 149 which lead to the reinjection system. The fly-ash enters the duct 14]: in a downward direction and fills the hopper bottom 142 and the lower portionof the duct 141. As soonas the level of the fly-ash reaches the branch conduits' 1-46 and 147, it spills over and isreinjected; if, however, the collection of fly-ash is more than can be conveniently handled by the reinjection system or by the furnace, the level continues to rise until the top of the sleeve 144 is reached. Then the excess fly-ash overflows into the standpipe 143 and is deposited in the ash pit or is stored for use at a time when the amount of fly-ash collection is not su-fficient to make full use of the reinjection system. v

Figure 11 shows still another limiting apparatus which may be used in the practice of the present invention. The apparatus 150 consists of a vertical duct 151 having a bottom member 152. Adjacent the bottom member is a generally horizontal duct 153 extending from the duct 151 to a conduit 154 which leads to the reinjection system. Another duct 155 extends horizontally from the duct 151 at a point somewhat higher in: level than the duct 153. This duct 155 leads to a conduit 156 leading to the ash pit or to a storage bin. The collected fly-ash falling downwardly intheduct 151 accumulates on' the bottom member 152 until thelevel reaches the duct 153; at that time the fly-ash overflows into the reinjection system. After the flow into' the duct 153' is as great as it can get, the level continuesto rise until it reaches the duct 155, whereupon the fly-ash overflows intotheduct 155 into the conduit 156 to the ashpit or storage bin. The size of the conduit 153 will of course be se'lected to limit the How of fly-ash therethrough to the. amount the reinjection system is capable of handling.

While certain novel features of the inventionhave been shown and described and are pointed out in the annexed claims, it will be understood that various omissions, substitutions, and changes in the forms and details of the device illustrated and in its' operation may be made by those skilled in the art without departing from the spirit of the invention.

The invention" having been thus described, what is 7 claimed as new and desired to secure by Letters Patent is:

1. Apparatus in combination with a steam generating unit having a combustion chamber, said apparatus com prising: a collector for receiving fly-ash from the products of combustion in said unit, means associated with said unit for disposing of ash, a fly-ash reinjection means connected to said unit for introducing into the combustion chamber fiy-ash received by said collector, a magnetic separator connected between the collector on the one hand and the disposal means and reinjection means on the other hand, the said magnetic separator permitting only the less magnetic particles of fly-ash to pass to the reinjection means.

2. Apparatus in combination with a steam generating unit having a combustion chamber, said apparatus comprising: a collector for receiving fly-ash from the products of combustion in said unit, an ash pit associated with said unit for disposing of ash, a fiy-ash reinjection means connected to said unit for introducing into the combustion chamber fly-ash received by said collector, a gravity separator connected between the collector on one hand and the ash pit and reinjection means on the other hand, said gravity separator permitting only the larger particles of fly-ash to pass to the reinjection means, a magnetic separator connected between the collector on the one hand and the ash pit and reinjection means on the other hand, the said magnetic separator permitting only the less magnetic particles of fiy-ash to pass to the reinjection means,

3. Apparatus in combination with a steam generating unit having a combustion chamber, said apparatus comprising: a collector for receiving fly-ash from the prod ucts of combustion in said unit, an ash pit associated with said unit for disposing of ash, fly-ash reinjection means connected to said unit for introducing into the combustion chamber fly-ash received by said collector, a magnetic separator connected between the collector on the one hand and the ash pit and reinjection means on the other hand, the said magnetic separator permitting only the less magnetic particles of fly-ash to pass to the reinjection means, a limiting apparatus connected between the collector on the one hand and the ash pit and reinjection means on the other hand, said limiting apparatus permitting only a predetermined amount of fiy-ash to pass to the reinjection means.

4. Apparatus in combination with a steam generating unit having a combustion chamber, said apparatus com prising: a collector for receiving fly-ash from the products of combustion in said unit, an ash pit associated with said unit for disposing of ash, fly-ash reinjection means connected to said unit for introducing into the combustion chamber fly-ash received by said collector, a gravity separator, a magnetic separator, and a limiting apparatus connected between the collector on the one hand and the ash pit and reinjection means on the other hand, said gravity separator permitting only the larger particles of fly-ash to pass to the reinjection means, the said magnetic separator permitting only the less magnetic particles of fiy-ash to pass to the reinjection means, said limiting apparatus permitting only a predetermined amount of flyash to pass to the reinjection means the gravity and magnetic separators being connected in series between the collector and the limiting apparatus.

5. Apparatus in combination with a steam generating unit having a combustion chamber, said apparatus comprising: a collector for receiving fly-ash from the products of combustion in said unit, an ash pit associated with said unit for disposing of ash, fly-ash reinjection means connected to said unit for introducing into the combustion chamber fly-ash received by said collector, a magnetic separator having an inlet duct connected to the collector, an outlet duct connected to the ash pit, and another outlet duct connected to the reinjection'means, said magnetic separator causing the less magnetic particles of fly-ash to pass into the outlet duct to the ash pit and the less magnetic particles to pass into the outlet duct to the reinjection means.

6. Apparatus in combination with a steam generating unit having a combustion chamber, said apparatus com prising: a collector for receiving fly-ash from the products of combustion in said unit, an ash pit associated with said unit for disposing of ash, fly-ash reinjection means connected to said unit for introducing into the combustion chamber fly-ash received by said collector, a gravity separator, and a magnetic separator, each separator having an inlet duct in communication with the collector, an outlet duct in communication with the ash pit, and another outlet duct connected to the reinjection means, said gravity separator causing the smaller particles of iiy-ash to pass intoits outlet duct to the ash pit and the larger particles to pass into its outlet duct to the reinjection means, said magnetic separator causing the more magnetic particles of fly-ash to pass into the outlet duct to the ash pit and the less magnetic particles to pass into the outlet duct to the reinjection means.

7. Apparatus incombination with a steam generating unit having a combustion chamber, said apparatus comprising: a collector for receiving fly ash from the products of combustion in said unit, an ash pit associated with said unit for disposing of ash, fly-ash reinjection means connected to said unit for introducing into the combustion chamber fly-ash received by said collector, a magnetic separator, and a limiting apparatus, the separator and the limiting apparatus each having an inlet duct connected to the collector, an outlet duct connected to the ash pit, and another outlet duct connected to the reinjection means, said magnetic separator causing the more magnetic particles of fiy-ash to pass into the outlet duct to the ash pit and the less magnetic particles to pass into the outlet duct to the reinjection means, the two outlet ducts of the limiting apparatus having means associated therewith to permit the duct leading to the reinjection means to contain the maximum flow of fly-ash that it can carry before fly-ash flows into the duct leading to the ash pit.

8. Apparatus in combination with a steam generating unit having a combustion chamber, said apparatus comprising: a collector for receiving fly-ash from the products of combustion in said unit, an ash pit associated with said unit for disposing of ash, fly-ash reinjection means connected to said unit for introducing into the combustion chamber fly-ash received by said collector, a gravity separator, a magnetic separator, and a limiting apparatus, the gravity separator, the magnetic separator and the limiting apparatus each having an inlet duct connected to the collector, an outlet duct connected to the ash pit, and another outlet duct connected to the reinjection means, said gravity separator causing the smaller particles of fly-ash to pass into its outlet duct to the ash pit and the large particles to pass into its outlet duct to the reinjection means, said magnetic separator causing the more magnetic particles of fiy-ash to pass into the outlet duct to the ash pit and the less magnetic particles to pass into the outlet duct to the reinjection means, the two outlet ducts of the limiting apparatus having means associated therewith to permit the duct leading to the reinjection means to contain the maximum flow of fly-ash that it can carry before fly-ash flows into the duct leading to the ash pit.

References Cited in the file of this patent UNITED STATES PATENTS 264,905 Spear et al. Sept. 26, 1882 302,513 Mertsheimer July 22, 1884 1,012,488 Wedge Dec. 19, 1911 1,417,189 McCarthy May 23, 1922 1,538,450 Stewart May 19, 1925 FOREIGN PATENTS I 627,759 Germany Mar. 23, 1936 627,866 Great Britain Aug. 17, 1949 

1. APPARATUS IN COMBINATION WITH A STEAM GENERATING UNIT HAVING A COMBUSTION CHAMBER, SAID APPARATUS COMPRISING: A COLLECTOR FOR RECEIVING FLY-ASH FROM THE PRODUCTS OF COMBUSTION IN SAID UNIT, MEANS ASSOCIATED WITH SAID UNIT FOR DISPOSING OF ASH, A FLY-ASH REINJECTION MEANS CONNECTED TO SAID UNIT FOR INTRODUCING INTO THE COMBUSTION CHAMBER FLY-ASH RECEIVED BY SAID COLLECTOR, A MAGNETIC 